EP2280919B1 - Method for producing acrolein by means of dehydration of glycerol - Google Patents

Description

The present invention relates to an improved process for producing acrolein by dehydration of glycerol in the presence of acid gas additives.

Acrolein is the simplest unsaturated aldehyde. It is also called 2-propenal, acrylaldehyde or acrylic aldehyde. By its structure, acrolein has a high reactive power through the presence of its two reactive functions that are likely to react individually or together. This is why acrolein has many applications, especially as a synthesis intermediate. In particular, it is a key intermediate for the synthesis of methionine, the synthetic amino acid used as a supplement to animal feed, which has become a substitute for fishmeal. Acrolein is a non-isolated synthesis intermediate of acrylic acid in the industrial production of acrylic acid by catalytic oxidation of propylene in the gas phase. The importance of the chemistry of acrylic acid and its derivatives is well known. Acrolein is also reacted with methyl vinyl ether and then hydrolyzed with glutaraldehyde, which has many uses in leather tanning, as a biocide in oil drilling and in the processing of cutting oils, and as a chemical disinfectant and sterilant for hospital equipment.

The most commonly used acrolein production method is based on the catalytic gas phase oxidation reaction of propylene with oxygen in the air. The acrolein thus obtained can then be directly integrated into a process for the manufacture of acrylic acid. When acrolein is used as a raw material for the synthesis of methionine and / or acrylic acid and / or acrylonitrile or for fine chemical reactions, a purification section makes it possible to eliminate the by-products of the reaction. mainly carbon oxides, acrylic acid, acetic acid and acetaldehyde.

The production of acrolein is therefore highly dependent on the propylene raw material obtained by steam cracking or catalytic cracking of petroleum fractions. This raw material, of fossil origin, contributes, moreover, to the increase of the greenhouse effect. It therefore appears necessary to have a process for the synthesis of acrolein that is not dependent on the propylene resource and that uses another, preferably renewable, raw material. This process would be particularly advantageous for the synthesis of methionine and other products, which could then be called "obtained from biomass". Indeed, methionine when it is used in animal feed is metabolized rapidly and the carbon dioxide that is found in the atmosphere contributes to the increase of the greenhouse effect. If acrolein is obtained from a renewable raw material, obtained for example from vegetable oil, CO ₂ emissions no longer enter the balance sheet of the process, because they offset the carbon dioxide used by biomass for its growth; there is no increase in the greenhouse effect. Such a process then meets the criteria associated with the new concept of "green chemistry" in a more global framework of sustainable development.

It is also known to synthesize aldehydes such as acrolein by dehydration of a polyhydric alcohol such as glycerol. Glycerol (also called glycerin when it is in the form of aqueous solution) is notably derived from the methanolysis of vegetable and animal oils at the same time as the methyl esters, which are themselves used in particular as fuels or fuels in diesel and fuel oil. domesticated. It is a natural product, available in large quantities, it can be stored and transported without difficulty. It has the advantage of being a renewable raw material meeting the criteria associated with the new concept of "green chemistry".

Many recent studies are devoted to the valorisation of glycerol and in particular to the preparation of acrolein. The process that is the subject of these studies implements a glycerol dehydration reaction according to the consecutive reactions:

CH ₂ OH-CHOH-CH ₂ OH → CH ₂ OH-CH ₂ -CHO + H ₂ O CH ₂ = CH-CHO + 2H ₂ O

which make it possible to obtain acrolein.

This reaction is a balanced reaction; as a rule the hydration reaction is favored at low temperatures, and dehydration is favored at high temperatures. To obtain acrolein, it is therefore necessary to implement a sufficient temperature, and / or a partial vacuum to move the reaction. The reaction can be carried out in the liquid phase or in the gas phase. This type of reaction is known to be catalyzed by acids.

According to the patent FR 69.5931 acrolein is obtained by passing glycerol vapors at a sufficiently high temperature over acid salts having at least three acidic functions such as, for example, phosphoric acid salts. The yields indicated are greater than 75% after fractional distillation.

In the patent US 2,558,520 the dehydration reaction is carried out in the gas / liquid phase in the presence of diatomaceous earths impregnated with phosphoric acid salts, suspended in an aromatic solvent. A conversion of glycerol to acrolein of 72.3% is obtained under these conditions.

The process described in the application WO 99/05085 is based on a complex homogeneous catalysis, under a CO / H ₂ atmosphere under 20/40 bar pressure and in the presence of a solvent such as an aqueous solution of sulpholane.

The Chinese patent application CN 1394839 relates to a process for preparing 3-hydroxypropanaldehyde from glycerol. The acrolein, an intermediate product of the reaction, is obtained by passing pure glycerol vaporized on a potassium sulfate or magnesium sulfate catalyst, and then the acrolein obtained is rehydrated to hydroxypropanaldehyde. The yields of the reaction are not given.

The patent US5,387,720 describes a process for the production of acrolein by dehydration of glycerol, in the liquid phase or in the gas phase, on acidic solid catalysts defined by their Hammett acidity. The catalysts must have a Hammett acidity of less than +2 and preferably less than -3. These catalysts correspond for example to natural or synthetic siliceous materials, such as mordenite, montmorillonite, acidic zeolites; supports, such as oxides or siliceous materials, for example alumina (Al ₂ O ₃ ), titanium oxide (TiO ₂ ), coated with inorganic acids, mono, di or triacids; oxides or mixed oxides such as gamma alumina, mixed oxide ZnO-Al ₂ O ₃ , or heteropolyacids. According to this patent, an aqueous solution comprising from 10 to 40% of glycerol is used and is carried out at temperatures of between 180 ° and 340 ° C in the liquid phase and between 250 ° and 340 ° C in the gas phase. According to the authors of this patent, the reaction in the gas phase is preferable because it makes it possible to have a glycerol conversion rate close to 100%, which leads to an aqueous solution of acrolein containing by-products. About 10% of the glycerol is converted to hydroxypropanone, which is found as a major by-product in the acrolein solution. Acrolein is recovered and purified by distillation or fractional condensation. For a reaction in the liquid phase, a conversion limited to 15-25% is sought to avoid an excessive loss of selectivity. The patent US 5,426,249 describes the same gas phase dehydration process of glycerol to acrolein, but followed by hydration of acrolein and hydrogenation to lead to 1,2- and 1,3-propane diol.

The dehydration reaction of glycerol to acrolein is thus generally accompanied by side reactions resulting in the formation of by-products such as hydroxypropanone, propanaldehyde, acetaldehyde, acetone, acrolein adducts on glycerol (called acetals), polycondensation products of glycerol, cyclic glycerol ethers ..., but also phenol and polyaromatic compounds which are at the origin of the formation of glycerol. coke on the catalyst. This results, on the one hand, in a reduction of the yield and in the selectivity of acrolein, on the other hand a deactivation of the catalyst. The presence of byproducts in acrolein, such as hydroxypropanone or propanaldehyde, some of which are more difficult to isolate, requires separation and purification steps which lead to high costs of recovery of the purified acrolein. Moreover, it is necessary to regenerate the catalyst very often so as to find a satisfactory catalytic activity.

The applicant company has sought to solve these problems by proposing in French Patent No. 2,882,052 to conduct the dehydration reaction of glycerol in the presence of molecular oxygen. It has been observed on this occasion that surprisingly the supply of oxygen reduces the formation of aromatic compounds such as phenol, and by-products from a hydrogenation of dehydrated products such as propanaldehyde and acetone, but also hydroxypropanone. The formation of coke on the catalyst is reduced. This results in inhibition of catalyst deactivation and continuous regeneration of the catalyst. Some by-products are present in much smaller amounts which facilitates subsequent purification steps.

Interestingly, these results are not economically sufficient to move to an industrial scale. On the other hand the implementation of the process in the presence of oxygen involves operating precautions to prevent it from racing to go to the combustion with its risk of explosion. it for example, the use of an inert gas to stay outside the flammable zone. Nitrogen in the air may be a part of this inert gas but will often be insufficient in quantity, which will lead to the use of additional inert gases such as recycle gases containing, in addition to nitrogen, which could not react, combustion gases and rare gases such as Argon, but also voluntarily added gases such as the gases mentioned above but also methane and light alkanes. The use of inert gases, which by definition do not contribute to the reaction, involves the use of a large reactor, compared to what is necessary for the reagents only. This entails an additional cost. This is the reason why the applicant company has continued its work to improve the acrolein selectivity of the reaction by focusing on the conditions of effectiveness and / or selectivity of the catalysts already known to be used for the synthesis of the acrolein from glycerol.

The applicant company has surprisingly discovered that acid catalysts known for the catalysis of the dehydration reaction which are homogeneous or solid multiphase materials insoluble in the reaction medium which, although acidic, could also have some undesirable sites likely to the origin of the formation of byproducts by reaction mechanisms sometimes unpredictable.

The aim of the present invention is to overcome these disadvantages by implementing the method by adding to the gaseous reaction medium a compound capable of being fixed, at least temporarily, to these sites and inhibiting them during the process to avoid the formation of by-products.

The present invention relates to a process for the synthesis of acrolein by dehydration of glycerol in the presence of a solid acid catalyst characterized in that it is carried out in a reaction medium comprising a gaseous phase containing an acidic compound.

For the purpose of the present invention, the term "acid compound" is intended to mean a compound which, in addition to what will be specified below, will have a pKa of less than 6.3 in solution in water. In particular, CO ₂ is not an acid within the meaning of the present invention.

The dehydration reaction is carried out, for example, on acidic solid catalysts such as those described in the French patent. FR 2,882,052 .

Suitable catalysts are homogeneous or multiphase materials which are insoluble in the reaction medium and which have a Hammett acidity, denoted H _{0 of} less than +2. As stated in the patent US5,387,720 which refers to the article of K. Tanabe et al in "Studies in Surface Science and Catalysis", Vol 51, 1989, Chapters 1 and 2 , the acidity of Hammett is determined by amine titration using indicators or by adsorption of a base in the gas phase. The catalysts meeting the acidity criterion H _{0 of} less than +2 may be chosen from natural or synthetic siliceous materials or acidic zeolites; inorganic carriers, such as oxides, coated with inorganic acids, mono, di, tri or polyacids; oxides or mixed oxides, iron phosphates or heteropolyacids.

Advantageously, the catalysts are chosen from zeolites, Nafion® composites (acid-based sulfonic fluorinated polymers), chlorinated aluminas, phospho or silico-tungstic and / or silicotungstic acids and salts, and various metal oxide solids such as tantalum oxide Ta ₂ O ₅ , niobium oxide Nb ₂ O ₅ , alumina Al ₂ O ₃ , titanium oxide TiO ₂ , zirconia ZrO ₂ , tin oxide SnO ₂ , silica SiO ₂ or silicoaluminate SiO ₂ Al ₂ O ₃ , impregnated with acidic functions such as borate BO ₃ , sulfate SO ₄ , WO ₃ tungstate, PO ₄ phosphate, SiO ₂ silicate, or MoO ₃ molybdate. According to the data of the literature, these catalysts all have a Hammett acidity H _{0 of} less than +2.

The preferred catalysts are sulphated zirconias, zirconium phosphates, tungsten zirconias, silicified zirconias, sulphated titanium or tin oxides, phosphated aluminas or silicas, doped iron phosphates, phospho or silico acid salts. -tungstiques.

These catalysts all have a Hammett acidity H _{0 of} less than +2, the acidity H ₀ can then vary to a large extent, up to values up to -20 in the reference scale with the Hammett indicators. The table given to the page 71 of the publication on acid-base catalysis (C. Marcilly) Vol 1 at Technip Editions (No. ISBN 2-7108-0841-2 ) illustrates examples of solid catalysts in this range of acidity.

The catalysts selected for this reaction are acidic solids. The acidity of solids can be measured in many ways and Hammett's method is just one of them.

The work of C. Marcilly referenced above also lists various methods for measuring the acidity and basicity of solids.

Aline Auroux's publications describe different methods of measuring solids acidity scales such as: A. Auroux and A. Gervasini, "J. Microcalorimetric Study of the Acidity and Basicity of Metal Oxide Surfaces" Phys. Chem., (1990) 94, 6371-79 and L. Damjanovic and A. Auroux, in Handbook of Thermal Analysis and Calorimetry, Vol 5, Chapter 11 pages 387-485: Recent Advances, Techniques and Applications, ME Brown and PK Gallager, editors (2008 Elsevier BV ).

Methods used to measure this acidity are described in the patents EP 1 714 696 [0038and 0039] and EP 1 714 955 [0045 and 0046] where we distinguish cases where the solid is white or not.

In particular, this work illustrates that a solid is rarely constituted either by only acid sites or only basic sites. The acidic solids have most of the time both acid sites, majority, but also some basic sites. This dichotomy is particularly illustrated in the article by A. Auroux A. Gervasini on page 6377 where Figure 13 shows that the same oxide can both adsorb an acid compound such as CO ₂ and a basic compound such as NH ₃ . Without wishing to be bound by any theory, it is thought that these contribute to the formation of by-products in the process.

The process is carried out in the presence of an acidic compound present in the gaseous phase of the reaction medium and which has an affinity with the basic undesirable sites constituting the catalyst. This compound will be selected from hard and soft acids as defined in the so-called Pearson classification. in the following articles: RG Pearson, J. Am. Chem. Soc., 85, 3533 (1963) ); RG Pearson, Science, 151 (1966) 172 ; RG Pearson, Chemistry in Britain, March 1967, 103 ; RG Pearson, J. Chemical Education, Vol 45 No. 9 (1968) 581 and Vol 45 No. 10 (1968) 643 ; RG Parr and RG Pearson, J. Am. Chem. Soc., (1983) 105, 7512 .

It should be pointed out that in Marcilly's book, referenced above on pages 34 and following, the scale based on Pearson's theory is used.

These compounds can be gaseous under normal conditions but they can be either liquid or even solid if they are likely to pass, under the operating conditions of the process, into the gaseous phase of the reaction medium.

Preferably, the dehydration is conducted in the presence of a gaseous phase containing a minor fraction of at least one acid compound as defined by Pearson's classification.

This acidic compound will be chosen especially from SO ₃ , SO ₂ , NO ₂ , etc. It would not be outside the scope of the invention, if a mixture of these compounds is used. According to Pearson's theory hard acids prefer to associate with hard bases and soft acids with moles. A mixture of compounds combining different acidities can be used to inhibit the various basic sites present on the catalyst.

The content of acidic compounds will depend on the nature of the catalyst chosen for the dehydration reaction. It will generally be between 1 and 3000 ppm of the gaseous phase or expressed in percentages by volume of 0.0001 to 0.3%.

If the reaction is carried out in the liquid phase, the acidic compound may be in liquid or even solid form if it is capable under reaction conditions of passing into the liquid phase to reach the above contents or, in the case of a solid compound , solubilize and then move to the liquid phase as has been specified above.

It should be noted that the patent EP 1 253 132 discloses the use in a process for synthesizing acrylic acid by oxidation of alkanes or acrolein in the presence of a reducing compound consisting of organic acids (formic or oxalic acid) or compounds containing sulfur such as SO ₂ or H ₂ S, SO ₂ being preferred. However, it may be emphasized that it is not the same reaction with a different catalyst and that the activity of said compound is to stabilize the catalyst and not to increase its selectivity. The reaction according to the invention can be carried out in the gas phase or in the liquid phase, preferably in the gas phase.

When the reaction is carried out in the gas phase, various process technologies can be used, namely fixed bed process, fluidized bed process or circulating fluidized bed process. In the first two processes, in a fixed bed or in a fluidized bed, the regeneration of the catalyst can be separated from the reaction.

It can be ex situ, for example by extraction of the catalyst and combustion in air or with a gaseous mixture containing molecular oxygen. In this case, the temperature and pressure at which the regeneration is effected need not be the same as those at which the reaction is carried out. Preferably the addition of the acidic compounds in the sense of Pearson, is performed at the Reactor and not during the regeneration.

According to the process of the invention, it can be carried out continuously in situ, at the same time as the reaction taking into account the presence of a small amount of molecular oxygen or a gas containing molecular oxygen in the reactor. In this case, the regeneration is similar to an inhibition of the deactivation and is done at the temperature and pressure of the reaction. Because of these particular conditions where the regeneration takes place continuously, the injection of the gaseous acid compound is found to be simultaneous and preferably upstream of the catalytic bed so that the acidic compounds are perfectly mixed in the reaction mixture.

In the circulating fluidized bed process, the catalyst circulates in two capacities, a reactor and a regenerator. It is known that the dehydration reaction is endothermic, so it is necessary to bring energy to the first capacity, while the regeneration consisting of the combustion of the coke is exothermic, it is necessary to extract calories from the second capacity. In the case of the circulating fluidized bed, the two systems can be compensated: according to the process of the invention, the regeneration of the catalyst under an oxygen flow by combustion leads to a heating of the catalyst and consequently provides the energy necessary for the dehydration reaction when the heated catalyst returns to the reactor. The residence time in each capacity depends on the deactivation rate of the catalyst and the coke rate formed on the catalyst. Indeed, a minimum coke rate is desirable to bring the solid back to the correct temperature, and a maximum coke rate is necessary to avoid degrading the solid by sintering during combustion. The injection of the gaseous acid compound is preferably carried out at the reactor.

The dehydration reaction is carried out in the gas phase in the presence of a catalyst at a temperature ranging from 150 ° C. to 500 ° C., preferably between 250 ° C. and 350 ° C., and a pressure of between 1 and 5 bar. When the reaction is carried out in the liquid phase in the presence of a catalyst at a temperature ranging from 150 ° C. to 500 ° C., preferably between 250 ° C. and 350 ° C., and a pressure greater than 5 bar and preferably comprised between between 20 and 80 bars.

The following examples illustrate the process of the present invention

When dehydrating glycerol in the presence of a conventional acidic catalyst is obtained acrolein but also by-products, such as hydroxypropanone, propanaldehyde, acetaldehyde, acetone, phenol, the products of addition of acrolein to glycerol, polycondensation products of glycerol, cyclic or non-cyclic glycerol ethers.

These examples will illustrate the impact of the presence of the acidic compound on the selectivity of the reaction with regard to the various known by-products and in particular of the hydroxypropanone which is the most present compound and is therefore representative of the efficiency of the process . They will also illustrate the impact of the presence of acid compounds on the deactivation of the catalyst.

Example 1

The reaction can be conducted under the following conditions. A Pyrex reactor containing a sintered catalyst bed is used. A catalyst such as the dehydration catalyst Zirconia Tungstea from Daiichi Kigenso KK, reference Z1044 having a mass of about 6.6 g and reduced to a particle size of 0.1-0.15 mm diluted with ml of fine grain silicon carbide (0.125 mm). Then we load a series of silicon carbide beds of different grain sizes: 2ml in 0.125 mm, 7 ml in 0.5 mm and finally 1.19 mm to the top of the reactor.

The reactor is then placed in an oven connected to the test facility. The temperature of the catalyst is regulated in temperature at 305 ° C. measured at the level of the "dehydration layer".

The reactor is fed from above by a gaseous mixture of Helium-Krypton / SO ₂ / Water-Glycerol at a pressure of 1.3 bar absolute. The Helium-Krypton gas mixture contains 4.92% Krypton which serves as internal standard. The water-glycerol mixture contains 30% by weight of glycerol.

• Hélium/Krypton/O₂/SO₂/ Eau/Glycérol : 50 / 2,6 / 3,4 / 0,02 / 40,6 / 3,4.

The constitution of the injected mixture is the following expressed in molar percentage:

• Helium / Krypton / O ₂ / SO ₂ / Water / Glycerol: 50 / 2.6 / 3.4 / 0.02 / 40.6 / 3.4.

The rate of introduction of the charge mixture is such that the hourly volume velocity (VVH) will be 2,000 h ^-1 . . The hourly space velocity is equal to the ratio of the total gas flow rate of the gas mixture expressed in normal liters per hour by the apparent volume of catalyst expressed in liters.

The effluents are trapped in the water leaving the reactor by a trap cooled to 0 ° C to separate the liquid effluents incondensables. Acrolein and hydroxypropanone, as the model compound of byproducts other than acrylic acid, are determined by chromatographic analysis.

The effluents are accumulated in the trap for a period of 60 minutes. Non-condensable gases are analyzed during the balance sheet. The yield of acrolein produced is 70 mol%, and acrylic acid 2 mol% and hydroxyacetone 0.5 mol%.

Example 2 (Comparative)

Example 1 will be repeated, but in the absence of SO ₂ .

The effluents are accumulated in the trap for a period of 60 minutes. Non-condensable gases are analyzed during the balance sheet. The yield of acrolein produced is 68 mol%, and acrylic acid 2 mol% and hydroxyacetone 2 mol%.

Example 3

The same Pyrex reactor is used as in Example 1. It is charged with a zirconia tungsten dehydration catalyst of Daiichi Kigenso Kagaku Kogyo reference Z1044 ring, crushed and sieved to a particle size of 0.32-0.50 mm. of volume 7 ml and mass 9.18 g. The undiluted catalyst is placed between two layers of silicon carbide.

The reactor is placed in an oven which is regulated at a temperature of 275 ° C. The reactor is powered by a gaseous mixture at 275 ° C N ₂ / O ₂ / SO ₂ / water / glycerol at a pressure of 1.3 bar absolute. This gaseous mixture is obtained by injecting into an electric vaporizer, on the one hand, a current of nitrogen and a stream of oxygen controlled in flow by mass flow controllers and, on the other hand, a liquid flow of a glycerol mixture (Prolabo ), demineralized water and 7.4% SO ₂ (Sigma-Aldrich) sulphurous acid by an HPLC type volumetric pump whose flow is controlled by a balance.

• N₂/O₂/SO₂/eau/glycérol : 15,4/3,9/0,005/74,5/6,2.

The constitution of the injected mixture is the following expressed in molar percentages:

• N ₂ / O ₂ / SO ₂ / water / glycerol: 15.4 / 3.9 / 0.005 / 74.5 / 6.2.

The charge introduction rate of the charge mixture is such that the hourly volume velocity (VVH) is 4,200 h ^-1

After 3 hours of injection of the gaseous mixture onto the catalyst, a material balance is carried out for 90 minutes in the same manner as in Example 1. The results are reported in Table 1.

Example 4

• N₂/O₂/SO₂/eau/glycérol : 15,4/3,9/0,025/74,5/6,2.

The conditions of Example 3 are reproduced with a gaseous mixture of molar composition:

• N ₂ / O ₂ / SO ₂ / water / glycerol: 15.4 / 3.9 / 0.025 / 74.5 / 6.2.

A balance is made after 3 hours and after 24 hours of injection of the mixture.

The results are reported in Table 1.

Example 5 (Comparative)

• N₂/O₂/SO₂/eau/glycérol : 15,4/3,9/ 0 /74,5/6,2.

The conditions of Example 3 are reproduced with a gaseous mixture of molar composition:

• N ₂ / O ₂ / SO ₂ / water / glycerol: 15.4 / 3.9 / 0 / 74.5 / 6.2.

A balance is made after 3 hours and 24 hours.

The results are reported in Table 1. <u> Table 1 </ u> Example 3 4 5 (comparative) Injection time (h) 3 3 24 3 24 SO ₂ (mol%) 0.005 0,025 0,025 0 0 Glycerol conversion (%) 100 100 87 100 69 Acrolein yield (%) 73 73 60 72 49 Yield hydroxypropanone (%) 0, 4 0.2 5, 9 2.4 5, 9

It is found that the addition of SO ₂ induces not only an improvement of the yield, but also limits the deactivation of the catalyst.

Claims (5)

1. A process for the synthesis of acrolein by dehydration of glycerol in the presence of a solid acid catalyst having a Hammett acidity of less than +2, characterized in that it is implemented in a reaction medium comprising a gas phase comprising a content between 1 and 3000 ppm of at least one acid compound within the meaning of the Person classification chosen from SO₃, SO₂ or NO₂.
2. The process as claimed in claim 1, characterized in that the catalyst is chosen from zeolites, Nafion® composites, chlorinated aluminas, phosphotungstic and/or silicotungstic acids and acid salts, and solids of the type comprising metal oxides, such as tantalum oxide Ta₂O₅, niobium oxide Nb₂O₅, alumina Al₂O₃, titanium oxide TiO₂, zirconia ZrO₂, tin oxide SnO₂, silica SiO₂ or silicoaluminate SiO₂/Al₂O₃, impregnated with acid functional groups, such as borate BO₃, sulfate SO₄, tungstate WO₃, phosphate PO₄, silicate SiO₂ or molybdate MoO₃.
3. The process as claimed in claim 1 or claim 2, characterized in that the catalyst is chosen from sulfated zirconias, phosphated zirconias, tungstated zirconias, silica zirconias, sulfated titanium or tin oxides, phosphated aluminas or silica, doped iron phosphates, or phosphor- or silicotungstic acid salts.
4. The process as claimed in one of claims 1 to 3, characterized in that the dehydration reaction is carried out in the gas phase at a temperature of between 150°C and 500°C, preferably of between 250°C and 350°C, and under a pressure of between 1 and 5 bar.
5. The process as claimed in one of claims 1 to 3, characterized in that the reaction is carried out in the liquid phase at a temperature of between 150°C and 500°C, preferably of between 250°C and 350°C, and under a pressure of greater than 5 bar and preferably of between 20 and 80 bar.